1. Field of the Invention
The present invention relates to an illuminance measuring method, an exposure apparatus using the method, and a device manufacturing method.
2. Description of the Related Art
With the recent remarkable progress in the fine processing technology for manufacturing semiconductor devices, it is demanded that an exposure apparatus be capable of forming a precise line which has a width identical to the wavelength of the exposure light used in the exposure apparatus. When the line is being formed, the presence of any illuminance irregularity in the illumination light (exposure light) on a reticle (mask) hinders a circuit pattern from being accurately projected onto a wafer.
To avoid this problem, conventionally, an illuminance distribution on the wafer is measured with the reticle removed from the optical path of the illumination light, and the illumination (exposure) system is adjusted so that an error measured in the illuminance distribution is set to be .+-.1% or less, thereby uniformity of illuminance on the reticle can be maintained.
However, even when uniformity of the illuminance on the reticle is maintained, in approximation with the reticle being removed from the optical path, a phenomenon called a "ghost" image occurs during actual exposure, because illumination light reflected by the reticle returns to the illumination system, and is further reflected inside of the illumination system, back to the reticle, again. To control the line width to a precision on the order of .+-.5% or less, it is necessary to control such a ghost image phenomenon, which has otherwise been ignored.
As a method for measuring such a ghost image, a method using a special reticle (e.g., a test reticle) as disclosed in, for example, Japanese Unexamined Patent Publication No. 5-62883, is known.
However, the above method is suitable only for presuming the amount of the ghost image that occurs, and disadvantageously, requires a significant measuring time due to its complicated procedure. In addition, the illuminance distribution on the reticle, caused by the ghost image phenomenon, varies depending upon the pattern density of the reticle or the shape of the reticle. This causes an inconvenience of preparing a test reticle for each pattern.